V.B.
I think you need to get your son into therapy sooner rather than later. I know you want to try to fix this yourself, but you really don't want to wait until he has "hit low" before you do something about it. If you do it sooner, you will see results sooner and you can possibly keep him and your family (other kids) from dealing with some really devastating things.
My opinion, and I'm no expert, is that your son feels like he has no control over his life right now. With your divorce and engagement, as well as another baby, he is probably just overwhelmed and is acting out to get some form of attention. Kids that age don't know how to deal with all of their feelings of confusion, anger, etc. Do yourself a favor and get him into some sort of therapy so that he can work this out before it gets worse.
Best of luck!